1. Field of the Invention
The present invention relates to a mobile communication system and, in particular to a method and apparatus for transmitting and receiving wireless backhaul subframe in a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM).
2. Description of the Related Art
OFDM is a multicarrier modulation scheme for transmitting data through multiple subcarriers in parallel. In an OFDM system, an input symbol stream is divided into several sub-symbol streams and modulated into multiple orthogonal subcarriers for transmission.
The origins of OFDM started in the late 1950's with the Frequency Division Multiplexing for military communication purpose, OFDM using orthogonal overlapping multiple subcarriers has been developed in 1970's but limited in wide spread used due to the difficult of implementing orthogonal modulations between multiple carriers. With the introduction of the idea of using a Discrete Fourier Transform (DFT) for implementation of the generation and reception of OFDM signals, by Weinstein, in 1971, the OFDM technology has developed rapidly. Additionally, the introduction of a guard interval at the start of each symbol and use of cyclic prefix (CP) overcomes the negative effects caused by multipath signals and delay spread.
Owing to such technical advances, the OFDM technology is applied in various digital communications fields such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). That is, the implementation of OFDM could be accomplished by reducing implementation complexity with the introduction of various digital signal processing technologies such as Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
OFDM is similar to Frequency Division Multiplexing (FDM) but much more spectrally efficient for achieving high speed data transmission by overlapping multiple subcarriers orthogonally. Due to the spectral efficiency and robustness to the multipath fading, OFDM has been considered as a prominent solution for broadband data communication systems.
Other advantages of OFDM are to control the Inter-symbol Interference (ISI) using the guard interval and reduce the complexity of equalizer in view of hardware as well as spectral efficiency and robustness to the frequency selective fading and multipath fading. OFDM is also robust to the impulse noise so as to be employed in various communication systems.
In wireless communications, high-speed, high-quality data services are generally hindered by the channel environments. In wireless communications, the channel environments suffer from frequent changes not only due to additive white Gaussian noise (AWGN) but also power variation of received signals, caused by a fading phenomenon, shadowing, a Doppler effect brought by movement of a terminal and a frequent change in a velocity of the terminal, interference by other users or multipath signals, etc. Therefore, in order to support high-speed, high-quality data services in wireless communication, there is a need to efficiently overcome the above channel quality degradation factors.
In OFDM, modulation signals are located in the two-dimensional time-frequency resources. Resources on the time domain are divided into different OFDM symbols, and are orthogonal with each other. Resources on the frequency domain are divided into different tones, and are also orthogonal with each other. That is, the OFDM scheme defines one minimum unit resource by designating a particular OFDM symbol on the time domain and a particular tone on the frequency domain, and the unit resource is called a Resource Element (RE). Since different REs are orthogonal with each other, signals transmitted on different REs can be received without causing interference to each other.
Physical channel is a channel defined on the physical layer for transmitting modulation symbols obtained by modulating one or more coded bit sequences. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a plurality of physical channel can be transmitted depending on the usage of the information sequence or receiver. The transmitter and receiver negotiate the RE on which a physical channel is transmitted, and this process is called mapping.
The LTE system is one of the representative system adopting the above-described OFDM in downlink while using Single Carrier-Frequency Division Multiple Access (SC-FDMA) in uplink.
FIG. 1 is a diagram illustrating a configuration of subframe for use in LTE (Long Term Evolution) system to which the present invention is applied.
Referring to FIG. 1, a given LTE transmission bandwidth 107 is segmented into a plurality of Resource Blocks (RBs), and each of RBs 109 and 113 is generated from 12 subcarriers in frequency domain and 14 OFDM symbols 113 or 12 OFDM symbols 121 in time domain and is a basic unit of resource allocation. A subframe 105 is has a length of 1 ms and consists of 2 slots 103.
The reference signal (RS) 119 is the signal agreed for use in channel estimation between a User Equipment (UE) and an evolved Node B (eNB) and transmitted through a corresponding antenna ports 0, 1, 2, and 3. Although the absolute positions of REs designated for RS in the frequency domain are configured differently depending on the cell, the relative interval between the RSs is maintained. That is, the RSs for the same antenna port are transmitted while spaced as many as 6 REs. The reason why the absolute positions of the RSs vary is to avoid collision between RSs in different cells.
Meanwhile, the control channel signal is transmitted at the beginning of the subframe in time domain. In FIG. 1, reference number 117 indicates the region in which the control channel signal is transmitted. The control channel signal can be transmitted across L OFDM symbols at the beginning of the subframe. L can be 1, 2, or 3. FIG. 1 is depicted to describe the case when L is 3. In case that one OFDM symbol is enough for transmitting the control channel, the first OFDM symbol of the subframe is assigned for the control channel (L=1). In this case, the rest 13 OFDM symbols are used for data transmission. The value L is used as the basic information for demapping at the receiver. Accordingly, if L not received, the UE cannot recover the control channel. In that a subframe is Multimedia Broadcast over a Single Frequency Network (MBSFN), the value of L is fixed to 2, and the LTE UE cannot receive the data region of the corresponding subframe.
The reason why the control channel signal is arranged at the beginning of the subframe is to allow a UE to check the control channel signal in advance to determine whether the data channel signal following the control channel signal is destined itself. That is, the UE determines whether to receive the data channel signal based on the control channel signal. If it is determined that there is no data channel signal destined to the UE, there is no need for the UE to receive the data channel signal and, as a consequence, the UE can save the unnecessary power consumption for receiving the data channel signal.
The LTE standard defines three downlink control channels, i.e. Physical Control Format Indicator Channel (PCFICH), Physical Hybrid ARQ Indicator Channel (PHICH), and Packet Data Control Channel (PDCCH), that are transmitted in unit of Resource Element Group (REG) 111 in the region 117 of FIG. 1.
PCFICH is the physical channel for transmitting the Control Channel Format Indicator (CCFI) to UE. CCFI is 2-bit long and indicates the number of symbols occupying the control region in a subframe “L”. Since a terminal can recognize the number of symbols of the control region based on the CCFI, the PCFICH must be the first channel to be received in a subframe except when the downlink resource is allocated persistently. Since UE does not know the value of L before receiving the PCFICH, the PCFICH is always mapped to the first OFDM symbol of each subframe. The PCFICH is transmitted in 4 resource groups formed by equally separating 16 subcarriers in frequency.
PHICH is the physical channel for transmitting downlink ACK/NACKs. PHICH is received by the UE which is performing uplink data transmission. Accordingly, the number of PHICHs is in proportion to the number of UEs performing uplink transmissions. PHICH is transmitted in the first OFDM symbol (LPHICH=1) or across three OFDM symbols (LPHICH=3) of the control region. The PHICH configuration information (number of channel, LPHICH) is broadcast through the Primary Broadcast Channel (PBCH) such that all of the UEs acquire the information at their initial connection to the cell. Also, PHICH is transmitted at predetermined position per cell like the PCFICH such that the UEs can acquire the PHICH configuration information by receiving the PBCH when the UE connects to the cell regardless of other control channel information.
In order to multiplex several ACK/NACK signals, Code Domain Multiplexing (CDM) technique is applied for PHICH. In a single REG, 8 PHICH signals are multiplexed into 4 real number parts and 4 imaginary number parts by means of the CDM technique and repeated as many as NPHICH so as to be distributed in frequency domain to obtain frequency diversity gain. By using NPHICH REG, it is possible to form the 8 or less PHICH signals. In order to form the PHICH signals more than 8, it is necessary to use other NPHICH REG.
After assigning resources for PCFICH and PHICH, the eNB determines the value of L, maps the physical channels to the REG of the assigned control region 117 based on the value of L. Next, the eNB performs interleaving to obtain frequency diversity gain. The interleaving is performed on the total REGs of the subframe determined by the value of L in unit of REG in the control region. The output of the interleaver in the control region is capable of preventing the Inter-Cell Interference (ICI) caused by using the same interleaver for the cells and obtaining the diversity gain by distributing the REGs of the control region across one or more symbols. Also, it is guaranteed that the REGs forming the same control channel are distributed uniformly across the symbols per control channel.
PDCCH 117 is a physical channel for transmitting data channel allocation information or power control information. The PDCCH 117 can be transmitted at different channel coding rates according to the channel condition of the target UE. Since Quadrature Phase Shift Keying (QPSK) is fixedly used for PDCCH transmission, it is required to change the resource mount for transmitting PDCCH 117 in order to change channel coding rate. When the channel condition of the terminal is good, a high channel coding rate is used to save the resource. In contrast, when the channel condition of the terminal is bad, a low channel coding rate is used to increase reception probability at the UE even with the cost of large amount of resource. The resource amount consumed by each PDCCH is determined in unit of Control Channel Element (CCE). Each CCE is composed of plural Resource Element Groups (REGs) 111. In order to secure diversity, the REGs of the PDCCH are arranged in the control region after interleaving has been performed.
Recently, researches are being conducted on LTE-Advanced (LTE-A) evolved from the legacy LTE. In the LTE-A system, the researches are focused on coverage expansion using relays to remove shadow area in the cell and wireless backhaul to establish wireless link between eNB and relay responsible for the same functions of the eNB.
FIG. 2 is a diagram illustrating a relationship between frames transmitted to and received from the relay in the OFDM-based system.
Referring to FIG. 2, the relay 203 receives eNB data from the macro eNB 201 and relays the eNB data to the UE attached to the relay. In the cell having a relay, there exists various types of links as shown in FIG. 2.
In FIG. 2, reference number 209 denotes Link A established between the eNB 201 and UE 207, and the UE 205 receives the data through Link C established with the relay 203. In view of the UE, since the relay is shown as a legacy eNB, Link A 209 and Link C 213 are regarded as identical transmission region as denoted by reference number 219. Reference number 211 denotes Link B established between the eNB and the relay so as to be used for the eNB transmit data to the UE connected to the relay and exchange higher layer signals between the eNB and relay. Reference numbers 215 and 233 denote the subframes carrying the data from the eNB to the relay and from the relay to the UE.
Reference number 215 denotes the configuration of a downlink subframe transmitted by the eNB to the UE or the relay, and reference number 217 denotes the configuration of a downlink subframe transmitted from the relay to the UE or received by the eNB. Reference number 219 denotes the configuration of a subframe transmitted from the eNB to the UE connected to the eNB or from the eNB or relay to the UE connected to the relay. Reference number 221 denotes the subframe carrying the data for backhaul transmission. The backhaul subframe can multiplex the transmission to the UE connected to the eNB depending on the scheduling and can be dedicated to the backhaul data transmission. Reference number 235 denotes the resource region for use in backhaul transmission.
The eNB transmits control channel 225 in all of the subframes, and the relay transmits controls channel in the same manner. However, since the relay cannot perform data reception and transmission simultaneously, it is cannot receive the control channel transmitted by the eNB while transmitting control channel. Meanwhile, the eNB transmits the data destined to the relay in the region 235 after control channel transmission, and the relay has to receive the information on the corresponding region. Since the relay has transmitted in the control channel transmission region, it is necessary to perform transmission/reception switching for receiving the data in the corresponding region in the blank region as denoted by reference number 229. The blank region 229 can be positioned at both ends of a subframe symmetrically or at one of both ends of the subframe asymmetrically.
FIG. 3 is a diagram illustrating configurations of backhaul subframe of the relay in the legacy LTE system.
Referring to FIG. 3, the configuration of backhaul subframe 307 is similar to the configuration for control and data channel transmissions of the legacy LTE system. In the legacy LTE subframe, a few symbols at the beginning of the subframe is used for transmitting control channels (PCFICH, PHICH, and PDCCH) 301, and the resource allocated for control channels are frequency multiplexed. In case of using this configuration, it is possible to transmit backhaul data for relay in some part of data transmission region. That is, the symbol including the region denoted by reference number 303 is used for relay control channel and the symbol including the region denoted by reference number 305 is used for relay data channel transmission.
This configuration following the legacy LTE subframe structure is advantageous in that additional channel coding and multiplexing scheme are not necessary. However, the region carrying the relay control channel should be assigned in advance as denoted by reference number 307, and scheduling the UEs connected to the eNB can be limited depending on the number of relays or assigned resource amount. In case that the allocated resource is larger than the resource for real transmission to the relay, resource waste becomes significant. The backhaul subframe as denoted by reference number 313 is configured to have a new relay control channel 311 and a relay data channel 311 and, the data amount to the relay is large, assigns additional relay data channel 315, unlike the conventional control channel configuration. This subframe configuration is advantageous in that allocated resource amount is smaller that of the subframe configuration denoted by reference number 307. However, it is a shortcoming that a new relay-dedicated control channel should be implemented.